On-line measurements made during the paper-making process generally include caliper (thickness), basis weight and moisture (e.g. % moisture). The measurements can be used for controlling process variables with the goal of maintaining output quality and thus minimizing the quantity of rejected product. The measurements are generally obtained at multiple locations across the paper sheet by scanning the sensor(s) in what is known as the cross direction (CD), or the measurements can be made at multiple locations down the length of the paper machine in what is known as the machine direction (MD). As described below, the measurements of caliper, basis weight and moisture content are conventionally made using three separate sensors/gauges.
Caliper measurements are generally either made by a device that physically contacts the sheet material or by non-physically contacting laser triangulation based device. Caliper sensors require access to both sides of the sheet. The contacting device is generally disliked because it can suffer from wear or build up issues, and can mark the sheet. The laser based device generally has a high degree of alignment tolerance requirements.
Basis weight sensors predominately use a nuclear radiation source and therefore are generally accompanied by regulatory issues. Like caliper sensors, basis weight sensors need access to both sides of the sheet.
Moisture measurement systems typically comprise infrared spectroscopy systems to measure the moisture content of the sheet. The spectroscopy system can operate either in transmission or reflection mode.
Requiring separate sensors/gauges for the measurements of caliper, basis weight and moisture has several disadvantages. One disadvantage is system cost and complexity. Another disadvantage is the inability to provide coincident measurements, where “coincident” as used herein refers to a plurality of different measurements made both at the same time and at the same location. When the measurements are not all made on the same paper location, errors can occur when using the respective measurements in combination to infer other information about the paper. For example paper on the scale of millimeters to centimeters can have relatively high variations in certain parameters due to the formation process. In the case of formation induced moisture variation, The moisture level of two adjacent spots on a paper sheet separated by 1 cm is known to be as high as 1%. When combining two measurements to calculate a third parameters such as basis weight and percent moisture to calculate the dry weight, If the two measurements are not made at the same location on the paper then an error in the dry weight calculated can result due to significant differences in moisture content at the respective measurement locations.